Currently, one major trend of WLED application is to replace existing traditional lamps. One problem to solve is to achieve smooth dimming of WLED with standard Triac dimmers which are conventionally designed for pure resistive lamp loads, such as incandescent or halogen light bulbs.
However, WLED does not appear as a resistive load to the Triac dimmer. Thus, when dimming WLED with conventional Triac dimmer, the dimming performance is often unsatisfactory. FIG. 1 illustrates a block diagram of a prior art driving circuit that applies the conventional driving system for a resistive lamp with Triac dimming to drive a WLED. The driving system 100 comprises: a Triac dimmer 101, an electronic transformer 103, a rectifier 105 and a WLED driver 107, for driving the WLED 109. Triac dimmer 101 regulates the power delivered from an AC power supply (usually 110V-220V) to the driving system 100 by monitoring the on time of its internal Triac, and outputs a high AC voltage having regulated conduction angles. A conduction angle represents the on time of said Triac in a cycle in degrees or radians.
Generally, a control signal is provided to turn on the Triac and a current will flow through it. When said current flowing through the Triac decreases to a determined value, the Triac turns off automatically. Electronic transformer 103 receives said high AC voltage and converts it into a low AC voltage. Rectifier 105 rectifies said low AC voltage and generates a low DC voltage to power said WLED driver 107 which drives the WLED in operation. As discussed in more detail below, several characteristics of the foregoing operation can cause the WLED to flicker. Accordingly, several improvements in circuits and methods for driving WLEDs may be desirable.